Highway mileage is where you see a pretty big difference---the efficiency and torque of the diesel allows it to cruise along consistently at little effort. City, I'd expect 2-4 mpg more from the diesel realistically.

With waverunners and your current N55, I doubt you'll see a difference of 1 mpg from day-today non-towing driving

Thanks for the helpful feedback. I'll stick with the N55 and save my money for other toys.